An organic electroluminescence element is a self-luminous display device and used for display or lighting. A display using an organic electroluminescence element is advantageous in terms of display performance such as high visibility and low viewing-angle dependency compared to conventional CRT or LCD. The display is also advantageous because it may be lighter and thinner.
In addition to advantages of reduced weight and thickness, there is a possibility that lighting of a shape which has not been able to be implemented may be implemented with an organic electroluminescence element.
As described above, an organic electroluminescence element has excellent features including those above. However, in general, layers constituting an organic electroluminescence element have a refractive index higher than air. For example, organic layers such as light-emitting layer in an organic electroluminescence element have a refractive index of 1.6 to 2.1. Thus, light emitted tends to be reflected totally at an interface. The light extraction efficiency is less than 20%, and most of the light is lost.
For example, a generally known organic electroluminescence element is configured with an organic layer allocated between a pair of electrode layers on a substrate. This organic layer includes a light-emitting layer, and the organic electroluminescence element extracts light emitted from this light-emitting layer and emits the light from a light extracting surface. In this case, it is not possible to extract the total reflection component, which is a light of the critical angle or more at the light-emitting surface or an interface between the electrode layer and the organic layer. Thus, there is a problem of low light extraction efficiency.
In order to solve such a problem, a method to improve light extraction efficiency has been proposed by employing a liquid-crystalline host material to control the shape of a light-emitting compound itself and orientation of the light-emitting compound (Non-patent literature 1).
In this proposal, however, a rod-like fluorescent material is used in order to increase polarization emission ratio, and Forster energy transfer (fluorescent resonance energy transfer) and reabsorption of emitted light occur due to uniaxial orientation thereof, which causes a problem of decreased light emission efficiency. Also, electrical exciton formation efficiency of the fluorescent material is low at about 25%. As a result, these is also a problem that external quantum efficiency cannot be improved.
To solve these problems, PTLs 1 to 3 propose a light-emitting element in which a phosphorescent material that no reduction of light emission efficiency due to Forster energy transfer occurs in principle is used as a phosphorescent light-emitting compound, which is oriented with a liquid-crystalline host material.
In these proposals, however, there is a problem that polarized emission ratio and luminescent quantum yield decrease.
Accordingly, it is the current situation that prompt development of an organic electroluminescence element which supports high luminous efficiency, high polarized emission ratio, high external quantum efficiency and high luminescent quantum yield at the same time is strongly desired.